This invention relates to a lead-free tin-base solder derived by melting a paste comprising a mixture of tin-bearing powders. More particularly, this invention relates to a paste that includes a powder formed of a relatively low melting tin alloy such that, upon heating during soldering operations, the alloy powder initially melts to wet the faying surface prior to melting of the bulk powder.
Solder pastes are employed to form a solder joint, for example, in physically and electrically connecting components of a microelectronic package. A typical paste comprises a powder formed of a solder alloy and dispersed in a liquid vehicle that contains a flux. One advantage of the paste is that it may be conveniently applied to the faying surface, for example, by screen printing. After the faying surfaces are arranged in contact with the paste, the arrangement is heated to melt the solder powder, whereupon the molten solder coalesces to form a liquid body sufficient to complete the connection. In order to obtain a strong bond, it is essential that the molten alloy flow in intimate contact onto the faying surfaces, which phenomenon is referred to as wetting. Wetting is enhanced by the flux, which reacts to remove oxides from the metal surfaces, particularly the faying surfaces. In a typical soldering step, fluxing occurs during the early stages of heating, so that the molten solder may readily flow onto the faying surfaces. Nevertheless, the time for heating must necessarily be sufficient to not only melt the solder powder, but also to permit fluxing and wetting to be completed.
Common solder paste includes a powder composed of a tin-lead alloy. During heating, the tin-lead alloy forms a eutectic liquid at a relatively low temperature. In order to accelerate heat transfer to the solder powder and thereby minimize the time at the elevated temperature, the assembly is typically exposed to an oven temperature that is as much as 40.degree. C. to 60.degree. C. above the solder melting point. Pastes are also available that contain a lead-free tin-base solder. In comparison to tin-lead solder, lead-free, predominantly tin solders tend to melt at relatively higher temperatures. Overheating significantly above the melting point, such as employed for tin-lead solder, tends to adversely affect other features of an electronic component package. It is also known to form a solder of an indium-tin or bismuth-tin alloy to take advantage of low melting eutectic phases. However, such low temperature solder is not adequate to withstand temperatures experienced during typical microelectronic applications. Thus, there is a need for a lead-free tin-base solder that permits soldering to be completed at a temperature suitable to avoid damage to microelectronic components, that minimizes the time required to melt the solder paste and wet the faying surfaces, and further that produces a solder joint that maintains its integrity despite temperature excursions of the type experienced by microelectronic packages during use.